In electrophotography, a photoreceptor in the form of a plate, belt, disk, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive layer, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas. A liquid or solid toner is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoreceptor. The resulting visible toner image can be transferred to a suitable receiving medium such as paper and film, or the photoreceptor surface can operate as a permanent receptor for the image. The imaging process can be repeated many times when a temporary or intermediate receptor is used.
The photoconductive element can be organic or inorganic. Both single layer and multilayer photoconductive elements have been used. In the single layer embodiment, a charge transport material and charge-generating material are combined with a polymeric binder and then deposited on the electrically conductive substrate. In the multilayer embodiment, the charge transport material and charge-generating material are in the form of separate layers, each of which can optionally be combined with a polymeric binder, deposited on the electrically conductive substrate. Two arrangements are possible. In one arrangement (the “dual layer” arrangement), the charge-generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge-generating layer. In an alternate arrangement (the “inverted dual layer” arrangement), the order of the charge transport layer and charge-generating layer is reversed.
A photoreceptor is required to have desired sensitivity and electrical properties depending on an electrophotographic process applied thereto. A photoreceptor subjected to repetitive uses is also required to have an excellent durability against electrical and mechanical forces applied thereto during corona charging, toner development, transferring to a receiving medium, and cleaning treatment. Furthermore, the surface layer of the photoreceptor may be contaminated by toners, and therefore it should have a good release property. Lastly, the surface of the photoreceptor should have good electroconductive properties so that charge will not remain on the surface of the photoreceptor after discharge to cause a background problem on prints.
For the surface layer of a photoreceptor to possess the above-mentioned desirable properties, photoreceptor may be provided with an overcoat to protect the photoconductive element. The typical overcoats comprise fluorinated polymer, siloxane polymer, fluorosilicone polymer, silane, polyethylene, polypropylene, polyurethane, polycarbonate, polyester, acrylated polyurethane, acrylated polyester, acrylated epoxide resin, or a combination thereof. Although these overcoats provide good abrasion resistance and durability, they are not electroconductive enough.
U.S. Pat. No. 4,006,020 to Polastri discloses an overcoated electrostatographic photoreceptor. The disclosed overcoating comprises a first polymer which is a terpolymer of methyl methacrylate, n-butylacrylate, and acrylic or methacrylic acid, and a second polymer which is a copolymer of styrene and maleic anhydride.
U.S. Pat. No. 3,753,709 to Staudenmayer et al. discloses overcoats for electrophotographic elements wherein the overcoats comprise a copolymer of vinyl acetate with a member selected from the group consisting of the alpha-beta ethylenically unsaturated carboxylic acids, which includes acrylic acid and methacrylic acid.
U.S. Pat. No. 4,181,526 to Blakey et al. discloses overcoats for electrophotographic elements wherein the overcoats comprise a terpolymer of methyl methacrylate, methacrylic acid, and 2-acetoacetoxyethyl methacrylate.
U.S. Pat. No. 4,062,681 to Lewis et al. discloses overcoats for electrophotographic elements wherein the overcoats comprise a polymeric composition such as a homopolymer, copolymer, or blend thereof and an alpha, beta-ethylenically unsaturated carboxylic acid or the partial alkyl ester thereof and at least 20% by weight of an organic cross-linking agent. An example of the overcoat is poly(methyl methacrylate-co-methacrylic acid) cured by an imine-terminated cross-linking agent.
U.S. Pat. No. 4,012,255 to McMullen discloses overcoats for electrophotographic elements wherein the overcoats comprise a terpolymer of 45 to 65 mole percent of methyl methacrylate, 25 to 40 mole percent of n-butylacrylate, and 5 to 15 mole percent of acrylic or methacrylic acid.
U.S. Pat. No. 4,734,347 to Endo et el. discloses overcoats comprising a fluorine-containing copolymer having monomer units of a fluoroolefin and methacrylic acid or acrylic acid.
U.S. Pat. No. 4,301,225 to Herrmann et el. discloses overcoats comprising copolymers of crotonic acid or maleic acid such as vinyl acetate-crotonic acid, vinyl acetate-maleic acid, and styrene-maleic acid.
However, in view of recent requirement of further improved image quality, a protective layer showing further improved properties in respects of electroconductivity, transparency, and durability is desired.